Conventionally, it was known that gas component can be separated through membranes made of polymer materials, since the polymer materials have an inherent gas permeability thereof. Recently, regarding environmental problem of global warming, such a membrane is suggested as a means for separating and recovering carbon dioxide from bulk carbon dioxide sources such as thermal power stations or cement plants, blast furnaces in steel mill and the like, while reducing energy. Meanwhile, natural gas or bio gas (gases generated by fermentation and anaerobic digestion of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbages, energy crops, and the like) is mainly a mixed gas of methane and carbon dioxide, and hydrogen is generally prepared via modification of natural gas with water vapor and water gas shift. In this process, a gas mixture containing about 40% of carbon dioxide and about 60% of hydrogen is obtained. When a method is capable of selectively permeating or removing carbon dioxide as impurity with a low concentration, the method may be considered to be economically excellent as a separation and purification method. Therefore, membrane separation methods have been researched as means for removing impurities (For example, see Patent document 1 and Patent document 2).
However, permeability (permeability coefficient) of gas in a polymer membrane is expressed by a product of gas solubility coefficient (solubility) and gas diffusion coefficient (diffusion property) in the polymer membrane (See, for example non-patent Document 1). For this reason, in order to selectively improve permeability (permeability coefficient) of carbon dioxide with respect to a separated gas, solubility coefficient (solubility) and/or diffusion coefficient (diffusion property) of carbon dioxide in polymer membranes are selectively improved. General polymer membranes are however known to have a problem of trade-off relation in which as selectivity increases, permeability decreases. In an attempt to overcome this trade-off of polymer membrane, a separation-active membrane, called “liquid membrane” or “facilitated transport membrane” is suggested.
The facilitated transport membrane is a membrane in which a substance (carrier) that reversibly and selectively reacts with only a specific permeation material is incorporated, which is also called “carrier transport membrane”. The permeation material can be permeated by a dissolution and diffusion mechanism through a membrane matrix as well as a formation of a reaction product with the carrier, and thus it is obtained a high permeation selectivity as compared to a co-existing gas other than permeation materials capable of being permeated by the dissolution and diffusion mechanism (see, for example, Non-patent Document 2). For example, in carbon dioxide/hydrogen separation, conventional polymer membranes have separation selectivity of about several tens (see, for example Non-patent Document 3). However, it is disclosed an example of the facilitated transport membranes having separation selectivity of 100 or higher (see, for example, Patent Documents 3 to 6).